In order to reducing a load of a spark plug, suppressing wasteful electric power consumption, and maintaining continuous spark discharge from the spark plug, an energy input circuit has been developed that is capable of maintaining the continuous spark discharge from the spark plug during, an optional period (hereinafter, continuous discharge period). The continuous spark discharge is generated by using a unidirectional current (secondary DC current) to a secondary coil by supplying electric energy to a battery voltage supply line from a low-voltage side of a primary coil before the main ignition of the spark plug is interrupted after first spark discharge (main ignition) is started by a known ignition circuit. (This technique is not a conventional technique, but a new technique.) A spark discharge (following the main ignition) continued by the energy input circuit will be referred to as the continuous spark discharge.
The energy input circuit maintains the spark discharge from the spark plug by adjusting the secondary current. The secondary current is adjusted by controlling a primary current (input energy) during the continuous discharge period. Adjusting the secondary current during the continuous discharge period can reduce a load of the spark plug caused by the repetition of the burn-out of the spark discharge and regeneration of the spark discharge. This further suppresses unnecessary electric power consumption, and provides continuous spark discharge from the spark plug. Further, because the secondary current flows in the same direction during the continuous spark discharge after the main ignition of the spark plug, it is possible to continue the spark discharge after the main ignition without the spark discharge being interrupted. For this reason, it is possible for the continuous spark discharge from the spark plug to avoid the burn-out of the spark discharge even if a swing flow of fuel and air is generated in a cylinder in a lean combustion.
Next, a description will be given of a typical example (to which the present invention is not applied) of an ignition device performing continuous spark discharge from the spark plug in order to recognize the concept of the present invention with reference to FIG. 24 to FIG. 26. (As previously described, the ignition device according to the present invention is a new technique different from a conventional technique.) The same components between the conventional technique shown in FIG. 24 to FIG. 26 and the following exemplary embodiments of the present invention will be referred as the same reference numbers and characters.
The ignition device shown in FIG. 24 has a spark plug, an ignition coil 3, a controller 4, and a signal transmission section. The controller 4 controls the execution of a main ignition and the continuous spark discharge from the spark plug. The signal transmission section transmits necessary signals to the controller 4. The controller 4 has a main ignition circuit 10 performing the main ignition and an energy supply circuit 11 to perform the continuous spark discharge.
The main ignition circuit 10 operates on the basis of an ignition signal IGT transmitted from an ECU 5 (engine control unit) as the signal transmission circuit. When the ignition signal IGT is switched from a low level to a high level, a current starts to flow in the primary coil of the ignition coil 3. After this, when the ignition signal IGT is switched from the high level to the low level, the current flowing in the primary coil is interrupted, a high voltage is generated in the secondary coil of the ignition coil 3, and the main ignition of the spark plug is initiated.
The energy supply circuit 11 operates on the basis of a discharge continuous signal IGW and a secondary current instruction signal IGA, which shows a secondary current instruction value I2a, transmitted from the ECU 5.
When the discharge continuous signal IGW is switched from a low level to a high level, the electric energy supply is started from a negative side (low voltage side) of the primary coil to a positive side (high voltage side) of the primary coil. In a concrete example, the secondary current is maintained at the secondary current instruction value I2a by turning on/off of an energy supply switch means.
Next, a description will be given of operation of the ignition device performing the continuous spark discharge with reference to FIG. 25. In FIG. 25, the label “IGT” indicates a high/low signal of the ignition signal IGT, the label “IGW” indicates a high/low signal of the discharge continuous signal IGW, the label “Ignition switch” indicates a turned ON/OFF operation of the ignition switching means, the label “Energy supply switch” indicates the turned ON and OFF operation of the energy supply switch means, the label “I1” indicates the primary current (current value flowing in the primary coil), and the label “I2” indicates the secondary current (current value flowing in the secondary coil).
When the ECU 5 transmits the ignition signal IGT, the ignition switch means is turned ON during a period ΔT1 (from t01 to t02) in which the ignition signal IGT is the high level.
When the ECU 5 outputs the ignition signal IGT, the ignition switching means is turned on during the period ΔT1 (from t01 to t02) in which the ignition signal IGT is at the high level.
After the start of the main ignition of the spark plug the secondary current is attenuated approximately in a saw tooth wave. The ECU 5 outputs the discharge continuous signal IGW before the secondary current is reduced to not more than a predetermined lower current value (to maintain the spark discharge).
When the ECU 5 outputs the discharge continuous signal IGB, the energy supply switch means is turned on and off to supply a part of energy accumulated in a capacitor in the energy supply circuit 11 to the primary coil. This makes it possible for the primary current to flow in the primary coil every turned on of the energy supply switch means. Further, the secondary current continuously flows in the secondary coil in the same direction of the secondary current flowing by the main ignition.
As previously described, the secondary current continuously flows to maintain the spark discharge by controlling the turning on and off of the energy supply switching means. That is, the secondary current is maintained within the predetermined target range (around I2a) during the period ΔT2 (from t03 to t04) in which the discharge continuous signal IGW is at the high level. As a result, the continuous spark discharge can be maintained in the spark plug during the high level of the discharge continuous signal IGW.
(Problems)
The ECU 5 transmits the ignition signal IGT to the main ignition circuit 10, and the discharge continuous signal IGW to the energy supply circuit 11. As shown in FIG. 26, each of the cylinders of the internal combustion engine requires the ignition signal IGT and the discharge continuous signal IGW. It is accordingly necessary for a four cylinder engine to use eight signal lines (four IGT#1 IGT#4 and four IGW#1 to IGW#4) through which the ECU 5 transmits to the controller 4 the ignition signal IGT and the discharge continuous signal IGW to the four cylinders of the engine.
In addition, when the secondary current instruction value I2a is varied according to the operation state of the engine, it is necessary to continuously transmit the secondary current instruction signal IGA to the energy supply circuit 11. This case requires an additional signal line to transmit the secondary current instruction signal IGA to the energy supply circuit 11. For example, as shown in FIG. 24, one of three current values (100 mA, 150 mA and 200 mA) is selected as the secondary current instruction value I2a according to the operation state of the engine. This case is required to use additional three signal lines of the three current values to transmit the secondary current instruction value I2a. 
As previously described, the ignition device capable of performing the continuous spark discharge uses additional signal lines to connect the ECU 5 with the controller 4. This structure of the conventional ignition device increases a manufacturing cost thereof.
(Technical References)
Patent document 1 discloses an ignition device having a circuit to perform a multiplex ignition in which a signal line to transmit an ignition signal IGT and a signal line to transmit a discharge continuous signal IGW are provided to each of cylinders of an engine.
Further, patent document 2 discloses a figure which shows one signal line to transmit the discharge continuous signal IGW, but does not show a multiple signal structure and a secondary current instruction value.